The invention concerns an endoprosthesis, in particular an intraluminal endoprosthesis such as a stent, having a carrier structure which entirely or in parts comprises a magnesium alloy.
The purpose of many endoprostheses is to implement a support function in the interior of the body of a patient. Accordingly, endoprostheses are designed to be implantable and have a carrier structure which ensures the support function. Implants of metallic materials are known. The choice of metals as the material for the carrier structure of an implant of that nature is based in particular on the mechanical properties of metals.
Metallic stents are known in large numbers. One of the main areas of use of such stents is permanently dilating and holding open vessel constrictions, in particular constrictions (stenoses) of the coronary vessels. In addition, aneurysm stents are also known, which afford a support function for a damaged vessel wall. Stents of that kind generally have a peripheral wall of sufficient carrying strength to hold the constricted vehicle open to the desired amount. In order to permit an unimpeded flow of blood through the stent it is open at both ends. More complicated configurations also permit an unimpeded flow of blood in side vessels (side branch access). The supporting peripheral wall is generally formed by a lattice-like carrier structure which makes it possible for the stent to be introduced in a compressed condition, when it is of small outside diameter, to the constriction to be treated in the respective vessel and there expanded, for example, by means of a balloon catheter, to such a degree that the vessel is of the desired enlarged inside diameter. Basically therefore, the stent is subject to the requirement that its carrier structure in the expanded condition affords a sufficient carrying strength to hold the vessel open. In order to avoid unnecessary vessel damage, it is also desirable that, after expansion and after removal of the balloon, the stent only slightly elastically springs back (recoil) so that upon expansion of the stent it has to be expanded only little beyond the desired final diameter. Further criteria which are desirable in relation to a stent include, for example, uniform surface coverage and a structure which allows a certain degree of flexibility in relation to the longitudinal axis of the stent.
In some cases, such as for example, in relation to screw means for complicated fractures or other connecting and supporting elements in bone surgery, suture materials or in particular in the case of intraluminal endoprostheses such as stents, a durable holding and support function afforded by the endoprosthesis is not required. Rather, in some of those situations of use, the body tissue can recover in the presence of the holding and support prosthesis in such a way that there is no need for an ongoing supporting action by the prosthesis. That has led to the idea of making such prostheses from bioresorbable material.
Besides the desired mechanical properties of a stent, as far as possible, it should interact with the body tissue at the implantation location in such a way that renewed vessel constrictions do not occur, in particular vessel constrictions caused by the stent itself. Re-stenosis (re-constriction of the vessel) should be avoided as much as possible. It is also desirable if the stent is, as far as possible, responsible for no, or only a very slight, inflammatory effect. In regard to a biodegradable metal stent, it is moreover desirable if the decomposition products of the metal stent as far as possible have no, or only very little, negative physiological effects and even positive physiological effects.
DE 197 31 021 discloses a bioresorbable metal stent, the material of which, as its main constituent, contains magnesium, iron or zinc. The mechanical properties, degradation behavior and biocompatibility mean that, in particular, magnesium alloys are to be preferred.
In DE 102 53 634, DE 101 28 100 or EP 1 395 297 the focus is on the use of such biodegradable magnesium alloys for medical purposes such as plates, screws, suture material or stents. The magnesium alloys have a magnesium content of over 70% by weight or over 90% by weight. With an increasing magnesium content however, the degradation time and therewith, the duration of the mechanical integrity required, rapidly decrease. In the case of stent uses, the degradation periods of such alloys are typically markedly less than 30 days. That is inadequate for many uses in medicine. Thus, in regard to stent uses, it has not hitherto been clearly established how long mechanical integrity is required to afford a sufficient supporting function. The estimates from experts vary from a few days to a year. In the case of complicated fractures, the healing process can easily involve 6 months.
Mechanical stability is also extremely important in particular for endoprosthesis uses. It permits the endoprosthesis to be of a compact design configuration while affording adequate stability. In the case of stents for example, the attempt is made to produce ever smaller leg widths as studies have shown that inter alia, the risk of re-stenosis is markedly reduced with the leg width as the leg inter alia mechanically irritates the surrounding tissue. Suitable material strengths are required to achieve that. Magnesium alloys which have been used hitherto, as set forth in DE 102 53 634, DE 101 28 100 or EP 1 395 297, are relatively soft. That limits the area of use as an endoprosthesis.
Besides the mechanical properties, biocompatibility of the alloy employed is essential for use as a medical implant. Alternative biodegradable materials such as for example, polymers, besides the poor mechanical properties, have to contend in particular with the difficulty of a low level of biocompatibility. Magnesium alloys have already exhibited markedly better properties, but it will be noted in this respect that this involves in particular aluminum-bearing alloys as described for example in DE 101 28 100 or EP 1 395 297. In that case, the aluminum is required inter alia for the formation of cover layers which are intended to slow down diffusion of the magnesium and thus the degradation process. According to those publications, that is required inter alia, in order to achieve sufficiently long mechanical stability for the endoprosthesis and to prevent outgassing phenomena in the degradation process.
Aluminum however, is known for causing damage to health, particularly when it is in ionic form. Thus, aluminum is known inter alia for causing damage to the central nervous system and triggering symptoms such as dementia, memory loss, loss of motivation or intense shaking. Aluminum is considered as a risk factor for Alzheimer's disease (Harold D Foster Ph D, Journal für Orthomolekulare Medizin 2/01). Adverse effects in regard to biocompatibility in the immediate proximity of endoprostheses comprising aluminum-bearing magnesium alloys could be detected in experiments. Thus, in animal experiments, pathological halos were observed around degrading legs of such stents as well as pronounced neointima hyperplasia, which counteracts the real purpose of the stent of preventing vessel closure. The use of aluminum in degradable medical implants such as in particular stents is thus not to be favored.
Hitherto, the approach in relation to medical implants involving activating the healing processes of the body itself, in the context of using endoprostheses, in order in that way further to improve the healing process, has been generally neglected.